Though its designed specifically for Metro, it also works on the classic Windows desktop environment. You can download it here, and it will be updated nightly. It's the same Firefox you know, with some added Windows 8/Metro-specific features. Here are a few to keep an eye out for:

Touch and swipe gesture support

Syncs with Firefox on the desktop

Supports Windows 8 charms

Data sharing between other Metro apps

Tab bar with tab previews

Keyboard shortcuts

View pages in desktop mode

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You can read the full list of features here. Since this is just a preview version, some of them might not show up in the full release, which we'll see at the end of October. [Mozilla via TechCrunch]